FIGS. 1A and 1B are diagrams illustrating a configuration of a related-art speaker apparatus.
The speaker apparatus 100 includes a diaphragm 103, a drive cone 105 which is disposed in back of the diaphragm 103 to be concentric with the diaphragm 103 and of which the inner circumference is air-tightly attached integrally to the inner circumference of the diaphragm 103, a frame 107 in which diaphragm supports 107a and 107b for supporting the outer circumferences of the diaphragm 103 and the drive cone 105 are fitted to the front end thereof and in which a magnetic circuit 109 is fitted to the central axis of the rear end thereof, and a voice coil 113 which is wound on a voice coil bobbin 111 bonded integrally to the inner circumference of the drive cone 105 and which is inserted into a magnetic gap 109a of the magnetic circuit 109.
The outer circumference of the diaphragm 103 is fixed to the diaphragm support 107a through an edge damper 115 bonded to the outer circumference thereof. The outer circumference of the drive cone 105 is fixed to the diaphragm support 107b through an edge damper 117 bonded to the outer circumference thereof.
The diaphragm support 107b is a step disposed at a position spaced apart by a proper distance in the axial direction from the diaphragm support 107a. An enclosed cylindrical wall not provided with any opening connects the diaphragm support 107a and the diaphragm support 107b of the frame to each other. The cylindrical wall 107c of the frame 107 closes an opening of a space 119 disposed between the diaphragm 103 and the drive cone 105 to form an enclosed space, and a vibration system including the voice coil 113 is supported by a spring property of air contained in the enclosed space 119.
The magnetic circuit 109 includes a yoke 121 in which a cylindrical center pole 121b is protruded from the center of a disk-shaped plate 121a, a ring-shaped magnet 123 which is inserted into the outer circumference of the center pole 121b, and a ring-shaped top plate 125 which is movably inserted into the front end of the center pole 121b so that the magnet 123 is inserted between the plate 121a and the top plate 125.
The gap between the inner circumference of the top plate 125 and the center pole 121b serves as a magnetic gap 109a in which the voice coil 113 is disposed.
The frame 107 has a shallow bawl shape and the magnetic circuit 109 is attached to the inner surface of the bottom portion 107d in a state that a plate 121a of a yoke 121 is placed thereon. That is, the magnetic circuit 109 is disposed between the rear end (bottom portion 107d) of the frame 107 and the drive cone 105.
An opening 107f as a vent hole for discharging back pressure of the diaphragm is properly formed between the bottom portion 107d of the frame 107 and the diaphragm support 107b. The rear surface of the diaphragm support 107a of the frame 107 serves as a flange surface which is closely fixed to a baffle of a speaker cabinet.
The speaker apparatus 100 is manufactured in the following procedure.
First, the magnetic circuit 109 is attached to the rear portion of the frame 107 and the voice coil 113 is temporarily built therein. Thereafter, the drive cone 105 is fixed to the diaphragm support 107b and then the diaphragm 103 is fitted and fixed to the diaphragm support 107a. The inner circumference of the diaphragm 103 is bonded and fixed to the inner circumference of the drive cone 105, thereby forming the enclosed space 119. The voice coil bobbin 111 is bonded and fixed to the inner circumference of the drive cone 105.
When the enclosed space 119 is completely formed, it is checked whether air tightness of the enclosed space 119 or a spring property as an air spring is proper. When it is proper, a dust cap 127 is attached and fitted to the center portion of the diaphragm 103, thereby completing the speaker apparatus. The dust cap 127 covers the front side of the voice coil bobbin 111 to prevent dust from invading the magnetic gap 109a. 
In the speaker apparatus 100, since the spring property as an air spring of the enclosed space 119 between the diaphragm 103 and the drive cone 105 controls vibration of the voice coil bobbin 111, the voice coil 113, the diaphragm 103, and the drive cone 105, it is not necessary to provide a specific damper for control.
Accordingly, the axial size of the speaker apparatus 100 is reduced due to omission of the specific damper and the installation space thereof, and it is thus possible to reduce the thickness of the speaker apparatus, which is required for a car audio system or the like.
Since the drive cone 105 can be made of the same material as the diaphragm 103, the weight is smaller and the mechanical fatigue is less generated, in comparison with the related-art bellows-shaped damper. Accordingly, it is possible to prevent deterioration in damping ability due to the mechanical fatigue of the constituent elements, thereby elongating the life time of the speaker apparatus. In addition, since large deformation is not locally generated, singular vibration or frictional sound making sound quality muddy does not occur. Accordingly, it is possible to reproduce sound with high quality without muddiness.
An electromotive speaker apparatus includes a magnetic circuit, a frame of which the back portion holds the magnetic circuit, a cone-shaped diaphragm of which a roll-shaped edge of the outer circumference is fixed to the front portion of the frame, and a voice coil wound on a cylindrical voice coil bobbin which is axially movable. The voice coil bobbin is elastically supported by the frame through a damper for regulating the axial movement. The damper generally has a bellows shape which is concentric about the voice coil bobbin (JP-A-63-155900 and JP-A-11-262085).
However, in the speaker apparatus of which a vibration system is supported by the damper, the length in the axial direction is large and the stiffness is great, thereby easily causing deterioration in support ability due to mechanical fatigue. As a result, there has been developed a damperless speaker apparatus in which an enclosed space is formed between a pair of diaphragms instead of the damper and the vibration system is supported by the spring property of air in the enclosed space.
FIGS. 12A and 12B illustrate a damperless speaker apparatus, where FIG. 12A is a longitudinal sectional view and FIG. 12B is a plan view of the speaker apparatus as seen from the front side.
The speaker apparatus includes a magnetic circuit 833 having a magnet 853, a frame 835 housing the magnetic circuit 833, a cylindrical voice coil bobbin 845 which is axially movable with respect to the magnetic circuit 833, a voice coil 846 wound on the voice coil bobbin 845 to face the magnetic circuit 833, a diaphragm 841 which is spaced axially apart from and concentric with the voice coil bobbin 845 and of which the outer circumference is fixed to the frame 835 and the inner circumference is fixed to the voice coil bobbin 845, and a drive cone 842 for delivering the drive power of the voice coil 846 to the diaphragm 841. A space surrounded with the diaphragm 841, the drive cone 842, and the frame 835 is enclosed air-tightly, and the diaphragm 841 and the drive cone 842 are connected to each other and reinforced by connection members 875 in the enclosed space 867. Reference numeral 823 denotes a dust cap disposed on the front surface of the speaker apparatus.
Thanks to the reinforcement by the connection members 875, the rigidity of the diaphragm 841 and the drive cone 842 is enhanced, thereby accomplishing improvement in propagation speed of acoustic vibration.
FIG. 16 is a diagram illustrating a configuration of a related-art electromotive speaker apparatus 1000, where the speaker apparatus includes a magnetic circuit 1003, a frame 1005 attached to the rear portion of the magnetic circuit 1003, a cone-shaped diaphragm 1009 in which a roll-shaped edge 1007 on the outer circumference thereof is fixed to a diaphragm support 1005a in front of the frame 1005, and a voice coil 1012 wound on a cylindrical voice coil bobbin 1011.
The magnetic circuit 1003 includes a yoke 1015 in which a cylindrical center pole 1015b is protruded from the center of a disk-shaped plate 1015a, a ring-shaped magnet 1016 which is inserted into the outer circumference of the center pole 1015b, and a ring-shaped top plate 1017 which is inserted into the front end of the center pole 1015b so that the magnet 1016 is inserted between the plate 1015a and the top plate 1017.
The gap between the inner circumference of the top plate 1017 and the center pole 1015b serves as a magnetic gap 1019 in which the voice coil 1012 is disposed.
An opening 1009a through which the voice coil bobbin 1011 is inserted is formed at the center of the diaphragm 1009. The edge 1007 bonded to the outer circumference of the diaphragm 1009 is fixed to the diaphragm support 1005a in the state that an attachment flange portion 1007a disposed in the edge 1007 is inserted between a ring-shaped gasket 1021 bonded to the diaphragm support 1005a and the diaphragm support 1005a. The inner circumference 1009b of the diaphragm 1009 having the opening 1009a is fixed to the voice coil bobbin 1011 inserted through the opening 1009a by adhesion.
A dust cap 1023 is disposed at the center of the diaphragm 1009. The dust cap 1023 covers the front side of the voice coil bobbin 1011 to prevent dust from invading a magnetic gap 1019.
The cylindrical voice coil bobbin 1011 is fitted to the outer circumference of the center pole 1015b to be axially movable and is elastically supported through a damper 1025 by the frame 1005 so as to regulate the axial movement.
Generally, as the damper 1025, a damper having a bellows structure concentric with the voice coil bobbin 1011 is widely used (for example, see JP-A-63-155900 and JP-A-11-262085).
The outer circumference of the damper 1025 is fixed to a damper support 1005b of the frame 1005 and the inner circumference thereof is fixed to the outer circumference of the voice coil bobbin 1011. Accordingly, the damper regulates the radial displacement of the voice coil bobbin 1011 and absorbs vibration energy by deformation of the bellows at the time of driving the diaphragm 1009, thereby controlling the vibration of the diaphragm 1009.
Generally, the frame 1005 has a structure that an opening 1005c is properly formed between the diaphragm support 1005a and the damper support 1005b. The opening 1005c serves as a vent hole for discharging back pressure at the time of driving the diaphragm 1009 and contributes to the decrease in weight of the frame 1005.
In the speaker apparatus 1000, when a sound signal is input to the voice coil 1012 through an input terminal and a wire not shown, the voice coil bobbin 1011 reciprocates to vibrate the diaphragm 1009, thereby reproducing sounds.
On the other hand, there has been also known a speaker apparatus having ability of reproducing low-frequency sound by reducing minus stiffness of means for giving the minus stiffness to the vibration system of the speaker apparatus (for example, see JP-A-2000-308174 and JP-A-2001-157290). In this case, by varying the stiffness of the means for giving the minus stiffness to the vibration system of the speaker apparatus, the minus stiffness is controlled, thereby changing the low-frequency characteristic.